Described below are a handoff preparation method in a radio communication system, particularly in a mobile radio system and an access device, a user station and a radio communication system able to carry out such a method.
One of the most important aspects of mobile communication systems is the continuity of mobility and services. Mobile communication systems according to GSM (Global System for Mobile Communication) or WCDMA (Wideband Code Division Multiple Access) and UMTS (Universal Mobile Telecommunications System) respectively offer complete mobility so long as a corresponding spatial coverage with network access devices is guaranteed. In contrast, systems according to WLAN (Wireless Local Area Network) only offer nomadic mobility, which means that no dedicated resource exists or is maintained in these systems whilst a user station communicating via this system is moving out of the area of the radio cell of the local data network.
In the case of the first-mentioned systems with the concept of complete mobility, it is required that the user stations carry out measurements of resources in adjacent cells during the communication in one radio cell or even carry out measurements in cells of other systems when a corresponding service is received. This is used for setting up a corresponding new connection via another cell or another resource in time before leaving the currently supplying cell. The measuring is performed in different manners in different systems, none of the concepts being suitable for extensions of the UMTS standard currently being developed, also known by the term 3G+. These extensions are based on, for example, an increase in usable bandwidths. In comparison with the UMTS networks currently used, in which frequency bands having a frequency width of 5 MHz are provided, frequency widths up to 20 MHz are provided for 3G+, possibly subdivided into a number of subfrequency widths of, for example, 1.25 MHz or 5 MHz. In this context, subscriber terminals will be able to support different frequency bandwidths depending on their technical equipment.
GSM is a system based on TDMA (Time Division Multiple Access) on the basis of the assignment of dedicated resources in the time domain in the uplink (UL) or downlink (DL) direction. Between the reception of data at the corresponding discrete instants of time, the user station can perform measurements. In addition, GSM provides a so-called super or hyper frame structure which also enables so-called multi-slot terminals, that is to say terminals having the capability of receiving simultaneously in a number of time slots, to have an almost continuous reception in order to perform measurements within certain predefined instants of time.
In contrast, UMTS is an FDD (Frequency Division Duplex) system in which the user station is continuously assigned dedicated resources in the uplink direction and in the downlink direction in the form of frequency channels. In order to perform measurements without a loss of data, a compressed mode was introduced. During short time intervals, so-called compressed mode gaps, the transmission of data in the uplink direction and/or in the downlink direction is interrupted which provides the user station with the possibility of performing measurements. In order not to lose some data during the transmission before and after the transmission gap in the compressed mode, the data rate is increased by reducing the spreading factor.
Correspondingly, sufficient time for carrying out measurements on other resources than the currently used resources is provided in the various mobile communication systems, other methods being known apart from the methods described. In these, for example, loss of a certain volume of data is accepted, i.e. the user station carries out measurements whilst the transmission of data to it is continued, and thus loses some data, data lost in this manner being compensated for by coding in higher layers or a new transmission of the data becoming necessary. It is also known to accept the loss of the data with a view to a reduced reproduction quality in the case, e.g. of transmitted voice. In addition, there exists the possibility of providing in the terminals or user stations a dual receiver equipment so that data reception is carried out continuously via a first receiver device whilst a second receiver device carries out the measurements on other resources.
The evolution of the UMTS standard (called 3G+ in the text which follows) is intended to offer complete mobility as a mobile communication system in the manner of a mobile radio system, in connection with which requirements were first specified which mandatorily provide the use of dual receivers. However, this would disadvantageously lead to increased costs of the subscriber terminals and to increased technical complexity. The use of user stations having only one receiver is therefore probable so that for such terminals, methods must be defined which provide for the support of handoffs across frequency bands. In this context, it is a matter of providing for measurements at different frequencies without having to accept the loss of data during the time of the measurement.
In addition, the problem with 3G+ is that this is a system with purely packet-switched data transmission which makes it more complicated to plan the transmission of data packets in comparison with so-called circuit-switched connections since it is intended to provide great possibilities with regard to the cell throughput on the basis of planning algorithms, using approaches according to the maximum throughput, round-robin etc. for this purpose. 2G and 3G are systems which are based on circuit switching and thus support a fundamentally different concept than 3G+ which is designed as a packet-switched system. In the circuit-switched systems, there are also packet-switched components such as HSDPA (High Speed Downlink Packet Access) and EDCH (Enhanced Dedicated Channel) but these are only additional options in order to improve the throughput. This can be seen from the requirements that a dedicated channel has to exist mandatorily in addition to the packet channel. However, 3G+ only has a pure packet channel for data transmission or dedicated data transmission so that the methods currently used for providing transmission or receiving gaps are not suitable for measurements since they are based on dedicated channels and are related to these. To perform measurements on other resources, active dedicated control channels are therefore always required and the user station has to continuously monitor the so-called scheduling channels for the packet-switched transmissions.